rf propagation no. 1 seattle pacific university basic rf transmission concepts
TRANSCRIPT
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RF Propagation No. 1Seattle Pacific University
Basic RF Transmission Concepts
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RF Propagation No. 2Seattle Pacific University
Radio Systems
Information Modulator Amplifier
Ant
Feedline
Transmitter
Information Demodulator Pre-Amplifier
Ant
Feedline
Receiver
Filter
Filter
RF Propagation
This presentation concentrates on the propagation portion
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RF Propagation No. 3Seattle Pacific University
Waves from an Isotropic source propagate spherically
• As the wave propagates, the surface area increases
• The power flux density decreases proportional to 1/d2
• At great enough distances from the source, a portion of the surface appears as a plane
• The wave may be modeled as a plane wave
• The classic picture of an EM wave is a single ray out of the spherical wave
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RF Propagation No. 4Seattle Pacific University
Real antennas are non-isotropic• Most real antennas do not
radiate spherically
• The wavefront will be only a portion of a sphere
• The surface area of the wave is reduced
• Power density is increased!• The increase in power density is
expressed as Antenna Gain
• dB increase in power along “best” axis
• dBi = gain over isotropic antenna
• dBd = gain over dipole antenna
Gain in this area
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RF Propagation No. 5Seattle Pacific University
Transmitted Power
• Radiated power often referenced to power radiated by an ideal antenna
ttGPEIRP Pt = power of transmitter
Gt = gain of transmitting antenna system
• The isotropic radiator radiates power uniformly in all directions
• Effective Isotropic Radiated Power calculated by:
Gt = 0dB = 1 for isotropic antenna
This formula assumes power and gain is expressed linearly. Alternatively,you can express power and gain in decibels and add them: EIRP = P(dB) + G(dB)
The exact same formulas andprinciples apply on the receiving side too!
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RF Propagation No. 6Seattle Pacific University
Propagation Models
• Large-scale (Far Field) propagation model
• Gives power where random environmental effects have been averaged together
• Waves appear to be plane waves
• Far field applies at distances greater than the Fraunhofer distance:
22Dd f D = largest physical dimension of antenna
= wavelength
• Small-scale (Near Field) model applies for shorter distances
• Power changes rapidly from one area/time to the next
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RF Propagation No. 7Seattle Pacific University
Propagation ModelsFor Free Space (no buildings, trees, etc.)
2
2
2
2 )4()4()(
c
fdd
P
PlinlossFree
r
t
dBdfc
fddBlossFree 56.147log20log20
4log10)( 1010
2
10
f = frequencyd = distance (m)= wavelength (m)c = speed of light
hb = base station antenna height (m)hm = mobile antenna height (m)a(hm) is an adjustment factor for the type of environment and the height of the mobile.
a(hm) = 0 for urban environments with a mobile height of 1.5m.Note: Hata valid only with d in range 1000-20000, hb in range 30-200m
)3)()(loglog55.60.44(
)(log82.13)6)((log16.2655.69)(
1010
1010
dh
hahfdBlossHata
b
mb
For Urban environments, use the Hata model
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RF Propagation No. 8Seattle Pacific University
Calculation of Received Signal Strength
1. Confirm that far-field metrics can be used: Use Fraunhofer distance
2. Calculate EIRP = Transmit Power * Antenna Gain3. Calculate propagation loss (free space or Hata)4. Received signal strength (RSS) = EIRP – propagation
loss
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RF Propagation No. 9Seattle Pacific University
Applying formulas to real systemsA transmission system transmits a signal at 960MHz with a power of 100mW usinga 16cm dipole antenna system with a gain of 2.15dB over an isotropic antenna.1. Confirm that far-field metrics can be used.
= 3.0*108 m/s / 960MHz = 0.3125 meters
Fraunhofer distance = 2 D2/ = 2(0.16m)2/0.3125 = 0.16m
2. Calculate EIRP.
Method 1: Convert power to dBm and add gainPower(dBm) = 10 log10 (100mW / 1mW) = 20dBmEIRP = 20dBm + 2.15dB = 22.15dBm
Method 2: Convert gain to linear scale and multiplyGain(linear) = 102.15dB/10 = 1.64EIRP = 100mW x 1.64 = 164mW
Checking work: 10 log10 (164mW/1mW) = 22.15dBm
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RF Propagation No. 10Seattle Pacific University
Applying formulas to real systemsA transmission system transmits a signal at 960MHz with a power of 100mW using a 16cm dipole antenna system with a gain of 2.15dB over an isotropic antenna.3. Calculate propagation loss (use free space in this example).
Loss(dB) = 20 log10(960MHz) + 20 log10(2000m) – 147.56dB
= 179.6dB + 66.0dB – 147.56dB = 98.0dB
Received power(dBm) = EIRP(dB) – loss = 22.15dBm – 98.0dB = -75.85dBmReceived power(W) = EIRP(W)/loss(linear) = 164mW / 1098.0dB/10 = 2.6 x 10-8 mW = 2.6 x 10-11 W Checking work: 10 -75.85dBm/10
= 2.6x 10-8 mW
What is the power received at a distance of 2km (use Hata model with base height 30 m, mobile height 1.5 m, urban env.)?Loss(dB) = 69.55+26.16(log(f)-6) – 13.82(log(hb)) – a(hm)+ [44.9-6.55(log(hb)](log(d)-3)
=69.55 + 78.01 – 20.41 – 0 + (35.22)(0.30) = 137.7 dB Received power = 22.15dBm – 137.7dB = -115.55dBm
4. Calculate RSS.
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RF Propagation No. 11Seattle Pacific University
Link Budget Analysis
Information Modulator Amplifier
Ant
Feedline
Transmitter
Information Demodulator Pre-Amplifier
Ant
Feedline
Receiver
Filter
Filter
RF Propagation
Gain
Gain
Loss
• A Link Budget analysis determines if there is enough power at the receiver to recover the information
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RF Propagation No. 12Seattle Pacific University
Transmit Power Components• Begin with the power output of the transmit amplifier
• Subtract (in dB) losses due to passive components in the transmit chain after the amplifier
• Filter loss• Feedline loss• Jumpers loss• Etc.
• Add antenna gain• dBi
• Result is EIRP
Information Modulator Amplifier
Ant
Feedline
Transmitter
Filter
RF Propagation
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RF Propagation No. 13Seattle Pacific University
Calculating EIRP
dBi12Antenna gain
dB(1.5)150 ft. at 1dB/100 footFeedline loss
dB(1)Jumper loss
dB(0.3)Filter loss
dBm4425 WattsPower Amplifier
ScaleValueComponent
dBm53Total
All values are example values
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RF Propagation No. 14Seattle Pacific University
Receiver System Components
InformationDemodulatorPre-Amplifier
Ant
Feedline
Receiver
Filter
• The Receiver has several gains/losses
• Specific losses due to known environment around the receiver• Vehicle/building penetration loss
• Receiver antenna gain
• Feedline loss
• Filter loss
• These gains/losses are added to the received signal strength
• The result must be greater than the receiver’s sensitivity
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RF Propagation No. 15Seattle Pacific University
Receiver Sensitivity• Sensitivity describes the weakest signal power level
that the receiver is able to detect and decode
• Sensitivity is dependent on the lowest signal-to-noise ratio at which the signal can be recovered
• Different modulation and coding schemes have different minimum SNRs
• Range: <0 dB to 60 dB
• Sensitivity is determined by adding the required SNR to the noise present at the receiver
• Noise Sources
• Thermal noise
• Noise introduced by the receiver’s pre-amplifier
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RF Propagation No. 16Seattle Pacific University
Receiver Noise Sources• Thermal noise
• N = kTB (Watts)• k=1.3803 x 10-23 J/K • T = temperature in Kelvin• B=receiver bandwidth
• Thermal noise is usually very small for reasonable bandwidths
• Noise introduced by the receiver pre-amplifier
• Noise Factor = SNRin/SNRout (positive because amplifiers always generate noise)
• May be expressed linearly or in dB
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RF Propagation No. 17Seattle Pacific University
Receiver Sensitivity Calculation• The smaller the sensitivity, the better the receiver
• Sensitivity (W) = kTB * NF(linear) * minimum SNR required (linear)
• Sensitivity (dBm) =10log10(kTB*1000) + NF(dB) + minimum SNR required (dB)
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RF Propagation No. 18Seattle Pacific University
Sensitivity Example
• Example parameters• Signal with 200KHz bandwidth at 290K• NF for amplifier is 1.2dB or 1.318 (linear)• Modulation scheme requires SNR of 15dB or 31.62 (linear)
• Sensitivity = Thermal Noise + NF + Required SNR• Thermal Noise = kTB =
(1.3803 x 10-23 J/K) (290K)(200KHz) = 8.006 x 10-16 W = -151dBW or -121dBm
• Sensitivity (dBm) = -121dBm + 1.2dB + 15dB = -104.8dB• Sensitivity (W) = (8.006 x 10-16 W )(1.318)(31.62) = 3.33 x 10-14 W
• Sensitivity decreases when:• Bandwidth increases• Temperature increases• Amplifier introduces more noise
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RF Propagation No. 19Seattle Pacific University
RSS and Receiver Sensitivity
• Transmit/propagate chain produces a received signal has some RSS (Received Signal Strength)
• EIRP minus path loss
• For example 50dBm EIRP – 130 dBm = -80dBm
• Receiver chain adds/subtracts to this
• For example, +5dBi antenna gain, 3dB feedline/filter loss -78dBm signal into receiver’s amplifier
• This must be greater than the sensitivity of the receiver
• If the receiver has sensitivity of -78dBm or lower, the signal is successfully received.
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RF Propagation No. 20Seattle Pacific University
Link Budget Analysis
Information Modulator Amplifier
Ant
Feedline
Transmitter
Information Demodulator Pre-Amplifier
Ant
Feedline
Receiver
Filter
Filter
RF Propagation
EIRP
Prop Loss
RSS
Sensitivity
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RF Propagation No. 21Seattle Pacific University
Link Budgets• A Link Budget determines what maximum path loss a system
can tolerate
• Includes all factors for EIRP, path loss, fade margin, and receiver sensitivity
• For two-way radio systems, there are two link budgets
• Base to mobile (Forward)
• Mobile to base (Reverse)
• The system link budget is limited by the smaller of these two (usually reverse)
• Otherwise, mobiles on the margin would have only one-way capability
• The power of the more powerful direction (usually forward) is reduced so there is no surplus
• Saves power and reduces interference with neighbors
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RF Propagation No. 22Seattle Pacific University
Forward/Reverse Link Budget Example
• Forward (Tower to Mobile)• Amplifier power 45dBm• Filter loss -2dB• Feedline loss -3dB• TX Antenna gain +10dB• Path loss X• Vehicle Penetration -12dB• RX Antenna gain +3dB• Feedline loss -3dB
• RSS at mobile = 38dBm – X (path loss)
• If Mobile Sensitivity is -100dBm• Maximum Path loss = 138dB
• Reverse (Mobile to Tower)• Amplifier power 28dBm• Filter loss -1dB• Feedline loss -3dB• TX Antenna gain +3dB• Vehicle Penetration -12dB• Path Loss X• RX Antenna gain +10dB• Feedline loss -3dB
• RSS at Tower = 22dBm – X (path loss)
• If Tower Sensitivity is -105dBm• Maximum Path loss = 127dB
Unbalanced – Forward path can tolerate 11dB more loss (distance) than reverse.Reduce Tower transmit power by 11dB.